Heat flux distributions and convective heat transfer in deep grinding

By using experimental data including the monitored temperature and power signals, combined with detailed theoretical analysis, the relationship between the undeformed grinding chip thickness and specific grinding energy has been studied and used to derive the heat flux distribution along the wheel-work contact zone. The relationship between the grinding chip thickness and specific grinding energy (SGE) has been shown to follow an exponential trend over a wide range of material removal rates. The distribution of the total grinding heat flux, q_t, along the grinding zone does not follow a simple linear form. It increases at the trailing edge with sharp gradients and then varies nearly linearly for the remainder of the contact length. The heat flux entering into the workpiece, q_w, is estimated by matching the measured and theoretical grinding temperatures, and it has been found that the square law heat flux distribution seems to give the best match, although the triangular heat flux is good enough for most cases to generate accurate temperature predictions. With the known heat flux distributions of q_t and q_w, the heat flux to the grinding fluid can then be estimated once the heat partitioning to the grinding wheel is determined by the Hahn model for a grain sliding on a workpiece. The convective heat transfer coefficient of the grinding fluid has been shown to vary along the grinding zone. An understanding of this variation is important in order to optimise the grinding fluid supply strategy, especially under deep grinding conditions when contact lengths are large. It has been demonstrated that the down grinding mode can provide a beneficial fluid supply condition, in which the fluid enters the grinding zone at the position of highest material removal where a high convective cooling function is needed.     

On the mechanics of the grinding process, Part III—thermal analysis of the abrasive cut-off operation

This is Part III of a 3 part series on the Mechanics of the Grinding Process. Part I deals with the stochastic nature of the grinding process, Part II deals with the thermal analysis of the fine grinding process and this paper (Part III) deals with the thermal analysis of the cut-off operation. Heat generated in the abrasive cut-off operation can affect the life of resin bonded grinding wheels and cause thermal damage to the workpiece. Thermal analysis of the abrasive cut-off operation can, therefore, provide guidelines for proper selection of the grinding conditions and optimization of the process parameters for improved wheel life and minimal thermal damage to the workpiece. In this investigation, a new thermal model of the abrasive cut-off operation is presented based on statistical distribution of the abrasive grains on the surface of the wheel. Both cutting and ploughing/rubbing that take place between the abrasive grains and the work material are considered, depending on the depth of indentation of the abrasives into the work material. In contrast to the previous models, where the apparent contact area between the wheel and the workpiece was taken as the heat source, this model considers the real area of contact, namely, the cumulative area of actual contacting grains present at the interface as the heat source. It may be noted that this is only a small fraction of the total contact area as only a small percentage of the abrasive grains present on the surface of the cut-off wheel are in actual contact with the workpiece at any given time and even a smaller fraction of them are actual cutting grains taking part in the cut-off operation. Since, the Peclet number, N_Pe in the case of cut-off grinding is rather high (a few hundred), the heat flow between the work and the contacting abrasive grains can be considered to be nearly one-dimensional. In this paper, we consider the interaction between an abrasive grain and the workpiece at the contact interface. Consequently, the heat source relative to the grain is stationary and relative to the workpiece is fast moving. The interface heat source on the grain side as well as on the workpiece side is equivalent to an infinitely large plane heat source with the same heat liberation intensity as the circular disc heat source. However, it will be shown in the paper that the contacting times are different. For example, the abrasive grain contacts the heat source, as it moves over the wheel-work interface, for a longer period of time ( milliseconds) whereas the workpiece contacts the heat source for shorter period of time ( a few microseconds). The temperature in the grinding zone is taken as the sum of the background temperature due to the distributed action of the previous active grains operating in the grinding zone (global thermal analysis) and the localized temperature spikes experienced at the current abrasive grain tip-workpiece interfaces (local thermal analysis), similar to the work reported in the literature [Proc Roy Soc (London) A 453 (1997) 1083]. The equivalent thermal model developed in the present investigation is simple and represents the process more realistically, especially the heat partition. The model developed provides a better appreciation of the cut-off operation; a realistic estimation of the heat partition between the wheel, the workpiece, and the chip; thermal gradients in the workpiece due to abrasive cut-off operation, and an insight into the wear of the cut-off wheels.     

GCr15轴套纵扭超声磨削表面形貌预测及试验研究

目的 构建纵扭超声磨削(LTUG)表面形貌预测模型,分析工艺条件和参数对表面粗糙度Ra值的影响,揭示GCr15轴套LTUG内圆表面形成机理。方法 基于弹性变形对LTUG单颗磨粒运动轨迹的影响,根据切削厚度概率密度函数和相邻磨粒轨迹重叠效应,建立了多磨粒超声振动作用下的最大未变形切削厚度模型,利用表面残余材料高度公式建立了GCr15轴套内圆磨削形貌预测模型,以LTUG和普通磨削(OG)方式对GCr15轴套内圆进行试验,采用正交试验验证表面形貌模型的准确性,观察并分析LTUG和OG作用后的GCr15轴套内圆表面形貌,最后利用所建立的表面形貌模型,研究磨削参数和超声振幅对表面Ra值的影响。结果 结果表明,基于所建立的表面形貌模型计算而得的表面Ra值与试验结果间的误差在13.2%以内,与OG相比,LTUG作用下的表面沟槽磨痕更均匀;LTUG作用下的表面轮廓呈现规律的周期性波动,且随振幅的增大,表面轮廓顶峰之间的间距逐渐增大;LTUG作用下的表面Ra值均低于OG,表面Ra值的降低幅度最大达到20%,随振动幅值的增大,表面Ra值逐渐减小,当振幅增大到一定程度时,表面Ra值呈现增大趋势。结论 建立的LTUG形貌预测模型具有良好的准确性,在合适的工艺参数下,LTUG可明显降低表面Ra值,与OG相比,在相同时间内,LTUG作用下的磨粒运动轨迹长度更长,且LTUG区域的弹性变形对磨粒运动轨迹和最大未变形切削厚度均有不同程度的影响。     

Characterization of N-doped polycrystalline diamond films deposited on microgrinding tools

Chemical vapor deposited diamond films have many industrial applications but are assuming increasing importance in the area of microengineering, most notably in the development of diamond coated microgrinding tools. For these applications the control of structure and morphology is of critical importance. The crystallite size, orientation, surface roughness, and the degree of sp ³ character have a profound effect on the tribological properties of the films deposited. In this article, we present experimental results on the effects of nitrogen doping on the surface morphology, crystallite size, and wear of microgrinding tools. The sp ³ character optimizes at 200 ppm nitrogen, and above this value the surface becomes much smoother and crystal sizes decrease considerably. Fracture-induced wear of the diamond grain is the most important mechanism of material removal from a microgrinding tool during the grinding process. Fracture occurs as a consequence of tensile stresses induced into diamond grains by grinding forces to which they are subjected. The relationship between the wear of diamond coated grinding tools, component grinding forces, and induced stresses in the model diamond grains is described in detail. A significant correlation was found between the maximum value of tensile stress induced in the diamond grain and the appropriate wheel-wear parameter (grinding ratio). It was concluded that the magnitude of tensile stresses induced in the diamond grain by grinding forces at the rake face is the best indicator of tool wear during the grinding process.     

An efficient method for solving the Signorini problem in the simulation of free-form surfaces produced by belt grinding

Industrial robots are recently introduced to the belt grinding of free-form surfaces to obtain high productive efficiency and constant surface quality. The simulation of belt grinding process can facilitate planning grinding paths and writing robotic programs before manufacturing. In simulation, it is crucial to get the force distribution in the contact area between the workpiece and the elastic contact wheel because the uneven distributed local forces are the main reason to the unequal local removals on the grated surface. The traditional way is to simplify this contact problem as a Signorini contact problem and use the finite element method (FEM) to calculate the force distribution. However, the FEM model is too computationally expensive to meet the real-time requirement. A new model based on support vector regression (SVR) technique is developed in this paper to calculate the force distribution instead of the FEM model. The new model approximates the FEM model with an error smaller than 5%, but executes much faster (1 s vs 15 min by FEM). With this new model, the real-time simulation and even the on-line robot control of grinding processes can be further conducted.     

砂轮表面磨粒出露高度对轴承钢磨削振动的影响

为探究砂轮表面磨粒形态对磨削振动的影响规律,提高磨削加工质量,构建了磨削振动模型并推导磨粒形态-接触刚性-磨削振动的对应关系,开展修整-磨削试验,通过试验分析并验证不同磨粒形态对磨削振动信号RMS和工件表面波纹特征W_a影响的差异。结果表明:在不影响砂轮锋利性的前提下,表征磨粒出露高度的砂轮AH值减小约58%,则RMS值和W_a值分别减小约47%和57%;在相同磨粒出露高度条件下,磨粒钝化的比例约20%,则RMS和W_a分别减小约22%和30%;同时,适度减小磨粒出露高度,磨粒适度钝化,有助于增大磨粒与工件接触面积,改善磨削振动,提高磨削加工质量。且提出的磨削振动模型与试验结果相符。      

金字塔砂带磨损状态的声信号GA-BP识别方法

目的 金字塔砂带连续磨损会引发钝峰、材料去除能力差和产热多等问题,为避免砂带磨损造成加工效率持续降低和工件表面质量逐渐恶化,需提高金字塔砂带磨损预测能力。方法 在配有声音采集系统的力控机器人磨削系统中对钛合金工件进行了砂带磨损试验;基于Archard模型建立了金字塔砂带磨损模型,并对金字塔砂带磨损程度进行量化;然后利用短时傅里叶和小波包分解分析、提取砂带磨损相关的声音特征;基于声音信号特征建立GA-BP模型,并对金字塔砂带磨损状态进行预测。结果 Kr与R0规律相近,随着磨削速度的增大而略微增大。对磨削声音进行小波包分解,DD2频段的声音特征随磨削时间逐渐降低,相较于其他频段更具有规律性。提取DD2频段的声音信号特征建立GA-BP模型,并对金字塔砂带磨损状态进行预测。结果表明,决定系数(R²)大于0.8,平均绝对误差(MAE)小于0.04,平均偏差误差(MBE)在±0.002之间,均方误差(RMSE)小于0.05。结论 随着砂带的磨损,金字塔尖锐的胞体开始磨平,单颗胞体的局部压力逐渐减小,材料去除能力减弱,产生的微振荡越来越弱,高频信号的声音特征逐渐下降。通过DD2频段声音信号特征建立的GA-BP模型对金字塔砂带磨损状态进行预测,具有准确性和稳定性。     

Simulation based optimization of the NC-shape grinding process with toroid grinding wheels

For achieving high material removal rates while grinding free formed surfaces, shape grinding with toroid grinding wheels is favored. The material removal is carried out line by line. The contact area between grinding wheel and workpiece is therefore complex and varying. Without detailed knowledge about the contact area, which is influenced by many factors, the shape grinding process can only be performed sub-optimally. To improve this flexible production process and in order to ensure a suitable process strategy a simulation-tool is being developed. The simulation comprises a geometric-kinematic process simulation and a finite elements simulation. This paper presents basic parts of the investigation, modelling and simulation of the NC-shape grinding process with toroid grinding wheels.     

Comparison of numerically and analytically predicted contact temperatures in shallow and deep dry grinding with infrared measurements

Contact zone thermal models of the grinding process are an important tool for the proper selection of process parameters to minimize workpiece damage while improving process efficiency. Validating contact zone thermal models with experimental measurements is difficult due to the high-speed and stochastic nature of the grinding process. In this work an infrared imaging system is used to validate two numerical thermal models, which are then compared to an established analytical contact zone thermal model. The two numerical thermal models consist of a shallow grinding model and a deep grinding model, where the deep grinding model takes the contact angle into account while the shallow grinding model does not. The results show that at small depths of cut both the numerical models and the analytical model perform well; however, as the depth of cut is increased the numerical models’ accuracy increases as compared to the analytical model. The increase in accuracy may be a result of the 2D solution of the numerical models as compared to the 1D solution of the analytical model. Additionally, it was found that the contact angle has very little effect on the contact temperatures. This work also reinforces Rowe's analytical work, using experimental and numerical results, which indicated that the workpiece temperatures are reduced by grinding at higher Peclet numbers for a given material removal rate.     

钛合金材料页轮磨抛材料去除深度模型

针对钛合金材料页轮磨抛表面去除量难控制的问题,从页轮磨抛的运动过程出发,结合Preston方程、Hertz接触理论及线接触变形等理论方法,建立页轮磨抛的材料去除深度模型。首先,分析磨削过程中页轮的运动和材料去除过程,并通过Preston方程简化磨抛过程,得到材料去除深度与页轮的线速度、接触压强、进给速度之间的关系;其次,通过Hertz接触理论及线接触变形得到接触压强和预压量的关系式,再将预压量代入接触压强,构建材料去除深度理论模型;最后,采用正交试验法和单因素试验法验证该理论模型的准确性,并通过极差分析法分析各参数对页轮磨抛去除深度的影响程度。结果表明:材料去除深度与预压量和页轮线速度成正比,与页轮进给速度成反比,且各参数对页轮磨抛去除深度的影响程度基本相当。模型预测的材料去除深度与试验结果的平均相对误差为6.25%,说明理论模型可准确预测磨削去除深度。      

A local process model for simulation of robotic belt grinding

A local process model to estimate the material removal rate in robotic belt grinding is presented and applied to the process simulation system. It calculate the acting force by incorporating the local geometry information of the workpiece instead of the cutting depth parameter with only one certain value as in a global grinding model. The simulation accuracy can be improved to below 5% even for a non-uniform contact under stable cutting conditions.     

Study on the grinding of P/M high speed steel ASP60

Several studies involved in grinding the P/M high speed steel ASP60 were carried out. The influence of the grinding mode under different grinding conditions was investigated. The grinding process was described and identified by using grinding parameters such as dimensionless maximum grain depth of cut h_g, arc of contact l_c, equivalent chip thickness h_eq. The optimum grinding parameters for ASP60 are suggested.     

Ultra-precision grinding of PZT ceramics—Surface integrity control and tooling design

A comprehensive statistical analysis of the factors controlling surface quality and form in ultra-precision grinding of polycrystalline lead zirconate titanate (PZT) ceramics has been conducted. The work focuses on practical grinding conditions and it includes an assessment of the interactions that exist between the method of material removal and the machine design. In the first phase of experimentation, defects including porosity and the fractural damage induced in the subsurface area were investigated. Machining trials were then conducted which were used to highlight the significant technical factors or combinations of technical factors that influence surface roughness, surface flatness and textural damage. A model for the systematic material removal mechanism which suggests that a relatively large depth of cut and ‘soft contact’ can be used to achieve improved surface integrity is proposed. In order to verify the suggested model, a series of design modifications to the tooling structure were made and the nature of the contact at the material removal interface was studied. Dramatic improvements in surface quality were achieved by incorporating a compliant polymer layer into the vacuum chuck used to hold the ceramics during grinding.     

轨道车辆转向架横梁非常规焊缝自动打磨系统及工艺研究

针对人工打磨轨道车辆转向架构架横梁非常规焊缝存在劳动强度大、质量低等问题,设计了一种轨道车辆转向架构架横梁非常规焊缝自动打磨系统,使用恒力控制装置(AFD)以控制打磨压力,使用激光扫描模块以确定焊缝的位置、高度等必要的打磨工艺信息,设计新的打磨刀具以便于机器人能够更好地对燕尾区及环形焊缝实施打磨作业,根据转向架构架横梁非常规焊缝的特点制定合理的打磨工艺。通过打磨实验,证明了所设计的轨道车辆转向架构架横梁非常规焊缝自动打磨系统的优越性以及打磨工艺的合理性。     

基于轴承毛坯表面分析的磨削材料去除率模型与应用实验

目的 在轴承套圈磨削加工中,传统基于动力学模型建立的磨削材料去除率模型仅考虑了磨削工件-砂轮-机床三者的弹性变形,未考虑毛坯零件表面不规则变形对模型的影响,导致传统理论模型在实际磨削应用中的效果不佳。针对此问题,基于轴承套圈毛坯表面形状分析建立了新的磨削材料去除率模型,并进行了应用实验。方法 基于轴承套圈毛坯零件表面形状的工艺研究,针对粗磨阶段毛坯零件表面不规则形状和弹性变形对磨削加工及产品质量的影响,建立不同偏心圆数量的轴承套圈结构分析方法,并提出一种以分段函数形式的磨削材料去除率模型,该模型充分考虑了轴承套圈毛坯零件表面不规则变形和偏心圆形状对磨削材料去除的影响,可有效反映轴承套圈实际材料磨削去除过程。最后,通过大量实验对所建的分段函数形式的磨削材料去除率模型进行应用实验研究。结果 与传统磨削材料去除率模型GPSM相比,所建的以分段函数形式的磨削材料去除率模型MMRG的准确率提高了96%以上,该模型可有效在线量化分析毛坯表面不规则大小及偏心圆结构。结论 该模型对指导毛坯零件制造,保证磨削加工质量和磨削加工效率有着重要的理论指导意义。     

An evaluation of grinding force by the fracture toughness of carbon steel

Grinding force, which is a representative factor of the grindability of a workpiece, is discussed in this study. The metallurgical structures of different carbon steels were classified into two groups according to the magnitude of their grinding force. For each group, the fracture toughness (J_c), was measured. The method used for this measurement was to mount a strain gauge on the hammer of a Charpy impact test machine. The strain at impact fracture was measured and the fracture toughness was calculated, with the results shown on a load-displacement diagram.A comparison of the fracture toughness (J_c) of each material and its grinding force was done. It was found that the fracture toughness (J_c) can be used in estimating the grinding force.     

Experimental and finite element analysis of temperature and energy partition to the workpiece while grinding with a flexible robot

Grinding processes performed with flexible robotic tool holders are very unlike conventional types of grinding because of low stiffness of the robot's structure. A special flexible robotic grinding process is used for in situ maintenance of large hydroelectric equipment for bulk material removal over large areas rather than as a finishing step, as is the case for most conventional grindings. Due to the low structural stiffness of tool holder, cutting is interrupted at each revolution of wheel during the grinding process. In this study, an investigation is carried out to determine the temperatures and energy partition to the workpiece for the above-mentioned flexible robotic grinding process by a three-dimensional finite element thermal model. Experiments were undertaken using embedded thermocouples to obtain the subsurface temperature at several points in the workpiece during the process. Then, energy partition to the workpiece was evaluated using a temperature-matching method between the experimental and numerical results. This ratio is used for predicting the temperature field at the wheel–workpiece interface with a relevant heat source function. Kinematics of cut and the flexible robot's dynamic behavior are considered in applying the heat input to the model. The energy partition to the workpiece in this specific flexible grinding process is found to be lower than for analogous conventional precision grinding processes. Two models, one from the literature and one from the power model of the process, are modified and proposed for determining the energy partition. The results showed that the energy partition ratio decreases by increasing the process power. Also, this ratio slightly decreases at higher feed speeds. In addition, lower temperatures were seen at higher powers due to the lower intensity of heat input over a larger contact area. Experimental observations show close agreement between simulated contact temperatures and measured results.     

Vitreous bond silicon carbide wheel for grinding of silicon nitride

This study investigates the grinding of sintered silicon nitride using a SiC wheel with a fine abrasive grit size and dense vitreous bond. The difference of hardness between the green SiC abrasive and sintered Si₃N₄ workpiece (25.5 vs. 13.7 GPa) is small. Large grinding forces, particularly the specific tangential grinding forces, are observed in SiC grinding of Si₃N₄. The measured specific grinding energy is high, 400–6000 J/mm³, and follows an inverse relationship relative to the maximum uncut chip thickness as observed in other grinding studies. The SiC wheel wears fast in grinding Si₃N₄. The G-ratio varies from 2 to 12. Two unique features in SiC grinding of Si₃N₄ are the trend of increasing G-ratio at higher material removal rate and the excellent surface integrity, with 0.04–0.1 μm R_a and no visible surface damage. For a specific material removal rate, surface cracks along the grinding direction are generated on the ground surface. The problem of chatter vibration was identified at high material removal rates. Periodic and uneven wheel loading marks and clusters of workpiece surface cracks across the grinding direction could be observed at high material removal rates. This study demonstrates that the SiC grinding wheel can be utilized for precision form grinding of Si₃N₄ to achieve good surface integrity under a limited material removal rate.     

Dynamic modeling of a novel workpiece table for active surface grinding control

In order to implement active control during surface grinding, a workpiece micro-positioning table utilizing a piezoelectric translator has been developed. The workpiece table has a working area of 100×100 mm and a nominal working range of 45 μm. The resolution of the workpiece table is less than 20 nm and the natural frequency is 579.2 Hz. The rigidity of the workpiece table under the open loop condition is approximately 17 N/μm. Under the closed loop condition, the rigidity is better than 400 N/μm. The design of the workpiece table is presented together with considerations to achieve the high dynamic characteristics, where the maximum acceleration is up to 33g (g is the acceleration of gravity). The dynamic models of the piezoelectric translator and the workpiece table have been established. Experimental testing has been carried out, verifying the performance of the workpiece table and the established dynamic models for the workpiece micro-positioning table.     

Prediction of contact conditions and theoretical roughness in manufacturing of complex implants by toric grinding tools

Throughout the world, more and more artificial knee prostheses are implanted due to the rise in cases of arthrosis. Inspired by all-ceramic hip joint implants, recent research in the field aims at enhancing the durability of the complex knee implants by using wear-resistant ceramics in a hard/hard joint pairing. Due to its kinematics and its complex geometry, the ceramic knee implant makes high demands on the manufacturing process if a high-quality level has to be assured. Firstly, the grinding process has to provide high surface qualities in order to reduce efforts in the polishing process; secondly, high contouring accuracy is required to avoid stress peaks in the ceramic joint pairings. Constant material removal in 5-axis grinding is assured by using toric grinding pins and accounting for the constantly changing contact conditions.In this paper, the contact conditions for a frontal grinding process with toric grinding pins are determined. Firstly, the contact length is calculated to detect the engagement zone in frontal grinding with toric tools. An angle range for constant material removal was found out. Furthermore, a model for the prediction of the resulting roughness of grinding processes with toric, ceramic bonded diamond grinding pins is developed and verified by means of grinding tests.